1. Technical Field
This invention relates to integrated optics devices, more particularly, it relates to an improved method for producing light-guiding structures in lithium niobate (LiNbO.sub.3) and lithium tantalate (LiTaO.sub.3) substrates.
2. Background and Prior Art
The operation of intergrated optics devices relies, inter alia, on the fact that electromagnetic radiation, e.g., optical or infrared radiation, can propagate through, and be contained by, layers of transparent materials. By forming such layers in appropriate substrates and shaping them into appropriate configurations, integrated optics technology has realized a variety of components which can perform a wide range of operations on optical radiation, e.g., guiding, modulating, deflecting, and filtering. See, for instance, S. E. Miller, "Integrated Optics: An Introduction," The Bell System Technical Journal, Vol. 48(7) pp. 2059-2069 (1969) and T. Tamir, Integrated Optics, Springer Verlag, New York, Heidelberg, Berlin (1975).
Materials that advantageously can serve as the basis for integrated optics devices include LiNbO.sub.3 and LiTaO.sub.3. These materials crystallize in the so-called trigonal crystal system which has a unique threefold symmetry axis, conventionally labeled the Z-axis or direction. The basal plane, i.e., the plane normal to the Z-direction, contains the unique X and Y directions, arranged at right angle to each other.
In general, optical radiation entering a crystal divides into two rays, called the ordinary ray and the extraordinary ray. These rays have polarization vectors at right angles to each other, and in general, they have different phase velocities, implying the existence of two refractive indices in such crystals, which are termed the ordinary refractive index n.sub.o and the extraordinary index n.sub.e.
A typical waveguide used in integrated optics is a strip waveguide, typically a thin and narrow region having somewhat higher refractive indices than the surrounding medium, with typical transverse dimensions of one to several wavelengths of the radiation. This last requirement translates into typical transverse dimensions of integrated optics strip waveguides of one to several micrometers. Such guiding structures are generally defined and produced by lithographic techniques akin to those used in integrated circuit technology.
Several methods exist for changing the refractive indices of LiNbO.sub.3 and LiTaO.sub.3, but since the one of interest to us in this application is in-diffusion of appropriate metal ions we will restrict our discussion to this method. A wide variety of elements, including Cu, Ni, Ag, Fe and Ge may be diffused into LiNbO.sub.3 or LiTaO.sub.3 to form guiding layers, and R. V. Schmidt and I. P. Kaminow, Applied Physics Letters, Volume 25(8), pp. 458-460 (1974), showed that one promising class of metals includes the transition elements, exemplified by Ti, V, and Ni. A thin layer, typically several hundred Angstroms thick, of an appropriate metal is deposited onto the substrate surface bounding the region to be transformed into a waveguide. Heating the sample, typically in flowing argon or other inert atmosphere or in oxygen, to temperatures above about 800.degree. C., typically (for Ti) in the neighborhood of 1000.degree. C., for a time of the order of one to several hours, brings about the diffusion of the deposited metal into the substrate, resulting in a diffusant profile that depends on such factors as diffusion temperature, diffusion time, nature and crystal orientation of the matrix, and nature and thickness of the diffusant layer. After completion of the diffusion step the samples are typically allowed to cool to room temperature in an atmosphere of flowing oxygen.
In-diffusion of appropriate metal ions into LiNbO.sub.3 and LiTaO.sub.3 results typically in an increase in both n.sub.o and n.sub.e, with the change in the refractive indices being typically proportional to the concentration of the in-diffused metal species. Thus, since in-diffusion of, e.g., Ti into LiNbO.sub.3 and LiTaO.sub.3 results in increases in both n.sub.o and n.sub.e, both the ordinary and the extraordinary ray can be guided in Ti-diffused strip wave guides by the usual mechanism of total internal reflection.
It has been known for some time that maintaining LiNbO.sub.3 or LiTaO.sub.3 at elevated temperatures, of the order of 1000 degress C., can result in out-diffusion of Li.sub.2 O. See, for instance, J. R. Carruthers et al, Applied Optics, Volume 13(10), pp. 2333-2342 (1974). It is reported there that such out-diffusion does not affect n.sub.o, but does increase the extraordinary index n.sub.e approximately linearly with decreasing concentration of Li.sub.2 O. Hence, such out-diffusion results in the creation of a surface layer having a higher n.sub.e than the unaltered substrate material. This surface region forms a guiding layer for the extraordinary ray, which is, in the geometries typically encountered in integrated optics, the ray that is polarized in the Z-direction. Such unwanted surface guides are typically formed during the process of in-diffusion of metal ions to form strip wave guides, and can adversely affect the operation of strip waveguides. In particular, since the strongest electro-optic coefficient for LiNbO.sub.3, r.sub.33, affects light of the extraordinary polarization, out-diffusion can become a significant problem in the fabrication of efficient optical waveguide devices.
The mechanism by which the above-described surface guiding layer can affect the operation of strip waveguides and related devices is the following. By raising the extraordinary index of refraction of the surface region of the substrate, and in particular in the substrate region adjacent to the nominal waveguide, the difference in n.sub.e between the strip guide and the surrounding substrate material is diminished, vanishes, or may even change sign, depending on the details of the situation. In any case, however, the ability of the strip waveguide to guide radiation polarized in the Z-direction will be diminished, resulting in leakage of radiation from the nominal waveguide to the surface guiding region. Such leakage not only constitutes a loss mechanism, but it also can lead to crosstalk between adjacent guides, and typically decreases the usefulness of Ti (and other metals) in-diffused strip waveguides for device applications. Thus, methods for avoiding or reversing such out-diffusion are of great interest, and a number of such methods have been found and applied.
One group of methods proposes to eliminate the depleted surface layer by means of in-diffusion of Li.sub.2 O. P. R. Ranganath and S. Wang, Applied Physics Letters, Volume 30(8), pp. 376-379, (1977) teach that, by carrying out the metal ion in-diffusion process in a gas stream, with chunks of Li.sub.2 O maintained upstream from the sample, the out-diffusion from the surface region can be avoided. This is thus a dynamical method, the success of which depends on maintaining at the sample surface some temperature-dependent minimum partial pressure of Li.sub.2 O during diffusion. In addition to requiring control of the vapor pressure, the method requires additional handling steps, and introduction of additional materials into the hot zone, with the attendant possibility of contamination of samples or equipment. S. Miyazawa, et al, Applied Physics Letters, Volume 31(11) pp. 742-744 (1977), teach that Li.sub.2 O out-diffusion can be suppressed by annealing the samples in Li.sub.2 CO.sub.3 powder at 600 degrees C. prior to the metal in-diffusion process. This method has the obvious disadvantage of requiring an additional processing step, and furthermore typically causes surface damage to the samples due to direct contact of the surface with the Li.sub.2 CO.sub.3 powder. B-U Chen and A. C. Pastor, Applied Physics Letters, Volume 30(11), pp. 570-571 (1977), annealed their LiNbO.sub.3 samples subsequent to the in-diffusion of Ti in LiNbO.sub.3 powder at 900 degrees C. for one hour or longer. This presumably led to a re-introduction of Li.sub.2 O into the depleted surface region, but the method is subject to the same shortcomings as that due to Miyazawa et al, supra.
Another approach to the control of out-diffusion is the carrying out of the in-diffusion step in a closed system. See R. J. Esdaile, Applied Physics Letters, Volume 33(8) pp. 733-734 (1978). This method relies on the establishment of equilibrium conditions early during the heat treatment, thereby minimizing loss of Li.sub.2 O from the sample. This is done in a manner that avoids some of the problems inherent in the analogous dynamical method due to Ranganath and Wang, op. cit., but does so at the cost of more involved apparatus and handling. J. Noda et al, Journal of Applied Physics, Volume 49(6), pp. 3150-3154, (1979) teach that by in-diffusing Mg into the depleted regions adjacent to the strip waveguide, the deleterious effects of Li.sub.2 O out-diffusion can be avoided, since the presence of Mg lowers n.sub.e in LiNbO.sub.3. This method, although effective, involves several additional and relatively complex processing steps and introduces new materials into the process.
In summary then, several methods exist for avoiding the deleterious effects of Li.sub.2 O out-diffusion from the surface region of LiNbO.sub.3 and LiTaO.sub.3 samples into which Ti or other elements have been diffused, but none of these methods appear to be wholly satisfactory. A simple, inexpensive, and reliable method for dealing with this out-diffusion problem would thus be of considerable technological interest, since it would make possible the manufacture of improved and more efficient waveguiding structures and other integrated optic devices in LiNbO.sub.3 and LiTaO.sub.3.